Exemplary embodiments of the present invention relate to a fuel cell system with at least one fuel cell.
PCT International Publication WO 2008/052578 A1 discloses a fuel cell system with a circulation system of anode waste gases of the fuel cell around an anode region thereof, which is described as a fuel circuit. It also discloses providing a drain line for draining liquid and gas from the region of the circulation system. This drain line, which removes liquid and gas, can thereby lead into the process air flow to a cathode region of the fuel cell.
The functionality of this structure is very good as a certain quantity of residual hydrogen is also present in the gas from the circulation system of anode exhaust gases besides the water and nitrogen enriching with time, the quantity of residual hydrogen thermally reacting on the catalysts in the cathode region of the fuel cell. Emissions of hydrogen into the environment of the fuel cell system are thus safely and reliably avoided.
In case of unfavorable load situations, for example a sudden load variation, in addition to the entry of a certain residual quantity of hydrogen into the cathode region, a comparatively large amount of water also enters into the region of the cathode. In particular, if the process air is already moistened by means of a moistener, which is frequently the case, this water will then be present largely in liquid form in the cathode region. An impairment in the performance of the fuel cell can then arise in that it blocks channels for the process air or wets corresponding surfaces. Individual cells of the fuel cell typically constructed as a cell stack can then exhibit power losses due to an under-supply of process air. In the worst case scenario pole reversal of individual cells can even arise so that the functioning of the fuel cell is impaired in the long term.
Exemplary embodiments of the present invention provide a fuel cell system with at least one fuel cell that avoids these disadvantages and is configured in such a way that it works reliably in all operating situations and with the best performance properties.
According to exemplary embodiments of the present invention the drain line opens into a line element for the process air flow, thus further before the cathode region of the fuel cell. However, this line is configured according to the invention so that it extends from a lower position into an upper position arranged higher when used properly in the direction of gravity. The flow of the process air thereby runs likewise from the lower position to the upper position. The drain line thus opens into an ascending pipe with upwardly orientated flow of the process air. This has decisive advantages. The gas coming from the drain line, typically hydrogen, is carried along in any case by the flow and goes into the cathode region of the fuel cell where it is thermally transformed in a known manner. This is guaranteed even with low flow speeds, for example during idling of the fuel cell system. If for any reason, for example the fuel cell system being switched off in the stop phase of a start/stop operation, no flow prevails in the line element and the hydrogen rises due to its low density nonetheless in the direction of gravity upwards and thus arrives in the region of the inlet to the cathode region of the fuel cell. The liquid, typically water, which has entered via the drain line into the line element is also drawn along in part by the flow in case of higher flow speeds and thus arrives—as has also been the case thus far—in the cathode region of the fuel cell. This is comparatively uncritical in case of a higher process air flow as in these situations a lower quantity of liquid in comparison with process air enters the cathode region of the fuel cell. A further part will in any case, also due to gravity, run down in the line element. The lower the flow speed of the process air flow the higher is the liquid portion which runs down in the line element and does not reach the cathode region of the fuel cell. If a certain flow speed of the process air flow is not reached all the liquid runs down in the line element.
With this structure of the fuel cell system according to the invention the entry of greater quantities of liquids or water into the cathode region of a fuel cell can be safely and reliably avoided so that the fuel cell exhibits a comparatively good and regular performance capacity across all operating situations.
This structure also allows the fuel cell system to be operated with high dynamics in relation to power without fearing massive power losses or the like. This predestines the fuel cell system with the structure according to the invention for use in a vehicle, in which it supplies, for example, the drive energy for the electric driving of the vehicle. Since particularly when driving vehicles highly dynamic power profiles are required the above-mentioned structure is particularly advantageous for such a use.
According to a particularly favorable and advantageous development of the fuel cell system according to the invention the line element extends at an angle of more than 45 degrees, in particular more than 80 degrees, with respect to the horizontal. The functionality according to the invention works in principle in case of any line element extending upwards at a corresponding angle. The steeper the angle the better the liquid can run downwards against the flow of the process air. A vertically upwardly extending line element or a line element extending upwards at least at an angle of more than 45 degrees or in particular more than 80 degrees would be ideal. As such, a line element is frequently easier to realize compared with a vertically upwardly extending line element due to the packaging of the fuel cell system, in particular in the abovementioned preferred use in a vehicle, a steeply upwardly extending pipe can of course be used which does not stand directly vertically on the horizontal.
In a further very favorable and advantageous embodiment of the fuel cell system according to the invention the drain line is formed in the region where it opens into the line element so that the liquid and gas that have entered reach the region of the walls of the line element. In particular, in case of incorporation of the gas and the liquid from the drain line into the region of the walls of the line element, the liquid can very easily run down in the region of the walls, as here—due to the friction between the flow of the process air and the walls—the flow speed is always somewhat lower.
In a further very favorable and advantageous embodiment of the fuel cell system according to the invention the cross-section of the line element is at least temporarily widened in the region where the drain line opens into the line element. Through such a widening of the flowable cross-section of the line element a reduction in the flow speed in the region of this process air is achieved—with a constant volume flow of the process air—due to the greater flowable cross-section. Through the reduction of the speed of the process air at least in the region in which the drain line opens into the line element very advantageous flow conditions can be created in order to encourage downward running of the liquid in the line element even with higher process air flow.
In a further very advantageous development of the fuel cell system according to the invention the drain line is formed in the region where the drain line opens into the line element so that the liquid and gas that have entered flow at least partially against the flow direction of the process air flow. Also with this structure, in which the substances are incorporated from the drain line contrary to or with a movement component contrary to the flow direction of the process air flow, a very advantageous effect is achieved with regard to downward running of the liquid in the line element. The substance mixture flowing out of the drain line into the region of the line element due to the pressure difference thus has a certain speed in the region of the inflow. Through a targeted inflow contrary to the speed of the process air flow, the speed of the inflowing substance mixture must first be reversed with energy from the process air flow before this can carry along the gas and in particular the liquid. Due to the very low density of hydrogen the latter undergoes in any case a reversal of its flow direction and is carried along. In case of rather heavy liquid droplets this will only be the case with higher flow speeds of the process air flow so that the majority of the liquid will run down or drip down contrary to the flow direction of the process air flow in the line element and here in particular in the region of the walls of the line element.
According to a very favorable embodiment of the fuel cell system according to the invention a container for collecting liquid is arranged in the flow direction of the process air before or in the region of the lower position. Such a container can receive the liquid running down in the line element and correspondingly store this so that the process air flow cannot carry along the collected liquid in the direction of the cathode region of the fuel cell. In an advantageous embodiment the container for collection of liquid is connected via a line to the exhaust air flowing out of the cathode region of the fuel cell. Through such a line, which can have either a diaphragm or a narrow point or even a valve, a connection of the container to the exhaust air from the fuel cell can be achieved. Thus the collecting liquid can be fed via the exhaust air of the fuel cell out of the fuel cell system in order to thus prevent entry or flow-through of the cathode region with the liquid.
According to a very advantageous embodiment of the fuel cell system according to the invention the line element is arranged between a moistener and the cathode region of the fuel cell, wherein the container for the collection of liquid and/or the line is/are formed integrated into the moistener. Fuel cell systems very frequently have moisteners in order to correspondingly moisten the process air after compression and before flowing into the cathode region. This is particularly important with fuel cells that are constructed as a stack of PEM fuel cells in order to protect the proton exchange membranes from drying out. Such moisteners are thereby frequently structured so that via a membrane, which is permeable to water vapor, the process air flow to the cathode region of the fuel cell is guided separately from the exhaust air flow from the cathode region of the fuel cell. As the exhaust air flow from the cathode region of the fuel cell is loaded with the product water produced in the fuel cell largely in vapor form, also partly in the form of droplets, this exhaust air flow can moisten the comparatively warmer and dryer process air flow to the cathode region of the fuel cell correspondingly. In such a moistener either the container and/or the line can be integrated. The structural resources can thereby be minimized as it is merely in the moistener that a connection provided with a throttle point or a valve means must be created between the exhaust air region and the container in the region of the process air.
According to a further very favorable embodiment of the fuel cell system according to the invention that the container is formed so that it can be heated. Liquid that has collected in the region of the container can thereby be vaporized due to the heating and thus serves for further moistening of the process air flow. The heating can thereby take place actively, for example, through an electric heating element or similar. With correspondingly robust membranes this structure also facilitates (as appropriate) the omission of a moistener as through the active vaporization of the collected liquid in the container a possibly sufficient moistening of the process air flow, which then arrives comparatively hot and dry in the region of the container, can be achieved.
In an alternative embodiment the container is in heat conducting contact with a heat generating component. The vaporization of the liquid collecting in the container can then serve to moisten the process air flow and also to at least partial cool the heat generating component in thermal contact with it, for example of an electric motor component, electronic power unit, or similar.